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authorElijah Newren <>2005-11-19 14:58:50 +0000
committerElijah Newren <>2005-11-19 14:58:50 +0000
commita7201d27d135fd113778bbde23aa9a8db83b8dfb (patch)
treedf04b6ddc82a1bbdf8ed8d913a4b7856f550f94c /doc
parentcfb3bd081aefa7922f5ed0adafdf01c903db7e86 (diff)
Merge of all the changes on the constraints_experiments branch. This is
2005-11-18 Elijah Newren <> Merge of all the changes on the constraints_experiments branch. This is just a summary, to get the full ChangeLog of those changes (approx. 2000 lines): cvs -q -z3 update -Pd -r constraints_experiments cvs -q -z3 diff -pu -r CONSTRAINTS_EXPERIMENTS_BRANCHPOINT ChangeLog Bugs fixed: unfiled - constraints.c is overly complicated[1] unfiled - constraints.c is not robust when all constraints cannot simultaneously be met (constraints need to be prioritized) unfiled - keep-titlebar-onscreen constraint is decoration unaware (since get_outermost_onscreen_positions() forgets to include decorations) unfiled - keyboard snap-moving and snap-resizing snap to hidden edges 109553 - gravity w/ simultaneous move & resize doesn't work 113601 - maximize vertical and horizontal should toggle and be constrained 122196 - windows show up under vertical panels 122670 - jerky/random resizing of window via keyboard[2] 124582 - keyboard and mouse snap-resizing and snap-moving erroneously moves the window multidimensionally 136307 - don't allow apps to resize themselves off the screen (*cough* filechooser *cough*) 142016, 143784 - windows should not span multiple xineramas unless placed there by the user 143145 - clamp new windows to screensize and force them onscreen, if they'll fit 144126 - Handle pathological strut lists sanely[3] 149867 - fixed aspect ratio windows are difficult to resize[4] 152898 - make screen edges consistent; allow easy slamming of windows into the left, right, and bottom edges of the screen too. 154706 - bouncing weirdness at screen edge with keyboard moving or resizing 156699 - avoid struts when placing windows, if possible (nasty a11y blocker) 302456 - dragging offscreen too restrictive 304857 - wireframe moving off the top of the screen is misleading 308521 - make uni-directional resizing easier with alt-middle-drag and prevent the occasional super annoying resize-the-wrong-side(s) behavior 312007 - snap-resize moves windows with a minimum size constraint 312104 - resizing the top of a window can cause the bottom to grow 319351 - don't instantly snap on mouse-move-snapping, remove braindeadedness of having order of releasing shift and releasing button press matter so much [1] fixed in my opinion, anyway. [2] Actually, it's not totally fixed--it's just annoying instead of almost completely unusable. Matthias had a suggestion that may fix the remainder of the problems (see [3] This bug was originally about not-quite-so-pathological cases but was left open for the worse cases. The code from the branch handles the remainder of the cases mentioned in this bug. [4] Actually, although it's far better there's still some minor issues left: a slight drift that's only noticeable after lots of resizing, and potential problems with partially onscreen constraints due to not clearing any fixed_directions flags (aspect ratio windows get resized in both directions and thus aren't fixed in one of them) New feature: 81704 - edge resistance for user move and resize operations; in particular 3 different kinds of resistance are implemented: Pixel-Distance: window movement is resisted when it aligns with an edge unless the movement is greater than a threshold number of pixels Timeout: window movement past an edge is prevented until a certain amount of time has elapsed during the operation since the first request to move it past that edge Keyboard-Buildup: when moving or resizing with the keyboard, once a window is aligned with a certain edge it cannot move past until the correct direction has been pressed enough times (e.g. 2 or 3 times) Major changes: - constraints.c has been rewritten; very few lines of code from the old version remain. There is a comment near the top of the function explaining the basics of how the new framework works. A more detailed explanation can be found in doc/how-constraints-works.txt - edge-resistance.[ch] are new files implementing edge-resistance. - boxes.[ch] are new files containing low-level error-prone functions used heavily in constraints.c and edge-resistance.c, among various places throughout the code. testboxes.c contains a thorough testsuite for the boxes.[ch] functions compiled into a program, testboxes. - meta_window_move_resize_internal() *must* be told the gravity of the associated operation (if it's just a move operation, the gravity will be ignored, but for resize and move+resize the correct value is needed) - the craziness of different values that meta_window_move_resize_internal() accepts has been documented in a large comment at the beginning of the function. It may be possible to clean this up some, but until then things will remain as they were before--caller beware. - screen and xinerama usable areas (i.e. places not covered by e.g. panels) are cached in the workspace now, as are the screen and xinerama edges. These get updated with the workarea in src/workspace.c:ensure_work_areas_validated()
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+File contents:
+ Basic Ideas
+ Important points to remember
+ Explanation of fields in the ConstraintInfo struct
+ Gory details of resize_gravity vs. fixed_directions
+IMPORTANT NOTE: There's a big comment at the top of constraints.c
+explaining how to add extra constraints or tweak others. Read it. I put
+that information there because it may be enough information by itself for
+people to hack on constraints.c. I won't duplicate that information in
+this file; this file is for deeper details.
+Basic Ideas
+There are a couple basic ideas behind how this constraints.c code works and
+why it works that way:
+ 1) Split the low-level error-prone operations into a special file
+ 2) Add robustness by prioritizing constraints
+ 3) Make use of a minimal spanning set of rectangles for the
+ "onscreen region" (screen minus struts).
+ 4) Constraints can be user-action vs app-action oriented
+ 5) Avoid over-complification ;-)
+Some more details explaining these basic ideas:
+ 1) Split tedious operations out
+ boxes.[ch] have been added which contain many common, tedious, and
+ error-prone operations. I find that this separation helps a lot for
+ managing the complexity and ensuring that things work correctly.
+ Also, note that testboxes.c thoroughly tests all functionality in
+ boxes.[ch] and a testboxes program is automatically compiled.
+ Note that functions have also been added to this file to handle some
+ of the tedium necessary for edge resistance as well.
+ 2) Prioritize constraints
+ In the old code, if each and every constraint could not be
+ simultaneously satisfied, then it would result in some
+ difficult-to-predict set of constraints being violated. This was
+ because constraints were applied in order, with the possibility for
+ each making changes that violated previous constraints, with no
+ checking done at the end.
+ Now, all constraints have an associated priority, defined in the
+ ConstraintPriority enum near the top of constraints.c. The
+ constraints are all applied, and then are all checked; if not all are
+ satisfied then the least important constraints are dropped and the
+ process is repeated. This ensures that the most important constraints
+ are satisfied.
+ A special note to make here is that if any one given constraint is
+ impossible to satisfy even individually (e.g. if minimum size hints
+ specify a larger window than the screen size, making the
+ fully-onscreen constraint impossible to satisfy) then we treat the
+ constraint as being satisfied. This sounds counter-intuitive, but the
+ idea is that we want to satisfy as many constraints as possible and if
+ we treat it as a violation then all constraints with a lesser priority
+ also get dropped along with the impossible to satisfy one.
+ 3) Using maximal/spanning rectangles
+ The constraints rely heavily on something I call spanning rectangles
+ (which Soeren referred to as maximal rectangles, a name which I think
+ I like better but I don't want to go change all the code now). These
+ spanning rectangles have the property that a window will fit on the
+ screen if and only if it fits within at least one of the rectangles.
+ Soeren had an alternative way of describing these rectangles, namely
+ that they were rectangles with the property that if you made any of
+ them larger in any direction, they would overlap with struts or be
+ offscreen (with the implicit assumption that there are enough of these
+ rectangles that combined they cover all relevant parts of the screen).
+ Note that, by necessity, these spanning/maximal rectangles will often
+ overlap each other.
+ Such a list makes it relatively easy to define operations like
+ window-is-onscreen or clamp-window-to-region or
+ shove-window-into-region. Since we have a on-single-xinerama
+ constraint in addition to the onscreen constraint(s), we cache
+ number_xineramas + 1 of these lists in the workspace. These lists
+ then only need to be updated whenever the workarea is (e.g. when strut
+ list change or screen or xinerama size changes).
+ 4) Constraints can be user-action vs app-action oriented
+ Such differentiation requires special care for the constraints to be
+ consistent; e.g. if the user does something and one constraint
+ applies, then the app does something you have to be careful that the
+ constraint on the app action doesn't result in some jarring motion.
+ In particular, the constraints currently allow offscreen movement or
+ resizing for user actions only. The way consistency is handled is
+ that at the end of the constraints, update_onscreen_requirements()
+ checks to see if the window is offscreen or split across xineramas and
+ updates window->require_fully_onscreen and
+ window->require_on_single_xinerama appropriately.
+ 5) Avoid over-complification
+ The previous code tried to reform the constraints into terms of a
+ single variable. This made the code rather difficult to
+ understand. ("This is a rather complicated fix for an obscure bug
+ that happened when resizing a window and encountering a constraint
+ such as the top edge of the screen.") It also failed, even on the
+ very example for which it used as justification for the complexity
+ (bug 312104 -- when keyboard resizing the top of the window,
+ Metacity extends the bottom once the titlebar hits the top panel),
+ though the reason why it failed is somewhat mysterious as it should
+ have worked. Further, it didn't really reform the constraints in
+ terms of a single variable -- there was both an x_move_delta and an
+ x_resize_delta, and the existence of both caused bug 109553
+ (gravity with simultaneous move and resize doesn't work)
+Important points to remember
+ - Inner vs Outer window
+ Note that because of how configure requests work and
+ meta_window_move_resize_internal() and friends are set up, that the
+ rectangles passed to meta_window_constrain() are with respect to inner
+ window positions instead of outer window positions (meaning that window
+ manager decorations are not included in the position/size). For the
+ constraints that need to be enforced with respect to outer window
+ positions, you'll need to make use of the extend_by_frame() and
+ unextend_by_frame() functions.
+ - meta_window_move_resize_internal() accepts a really hairy set of
+ inputs. See the huge comment at the beginning of that function.
+ constraints gets screwed up if that function can't sanitize the input,
+ so be very careful about that. It used to be pretty busted.
+Explanation of fields in the ConstraintInfo strut
+As of the time of this writing, ConstraintInfo had the following fields:
+ orig
+ current
+ fgeom
+ action_type
+ is_user_action
+ resize_gravity
+ fixed_directions
+ work_area_xinerama
+ entire_xinerama
+ usable_screen_region
+ usable_xinerama_region
+A brief description of each and/or pointers to more information are found
+ orig
+ The previous position and size of the window, ignoring any window
+ decorations
+ current
+ The requested position and size of the window, ignoring any window
+ decorations. This rectangle gets modified by the various constraints
+ to specify the allowed position closest to the requested position.
+ fgeom
+ The geometry of the window frame (i.e. "decorations"), if it exists.
+ Otherwise, it's a dummy 0-size frame for convenience (i.e. this pointer
+ is guaranteed to be non-NULL so you don't have to do the stupid check).
+ action_type
+ Whether the action being constrained is a move, resize, or a combined
+ move and resize. Some constraints can run faster with this information
+ (e.g. constraining size increment hints or min size hints don't need to
+ do anything for pure move operations). This may also be used for
+ providing slightly different behavior (e.g. clip-to-region instead of
+ shove-into-region for resize vs. moving operations), but doesn't
+ currently have a lot of use for this.
+ is_user_action
+ Used to determine whether the action being constrained is a user
+ action. If so, certain parts of the constraint may be relaxed. Note
+ that this requires care to get right; see item 4 of the basic ideas
+ section for more details.
+ resize_gravity
+ The gravity used in the resize operation, used in order to make sure
+ windows are resized correctly if constraints specify that their size
+ must be modified. Explained further in the resize_gravity
+ vs. fixed_directions section.
+ fixed_directions
+ There may be multiple solutions to shoving a window back onscreen.
+ Typically, the shortest distance used is the solution picked, but if
+ e.g. an application only moved its window in a single direction, it's
+ more desirable that the window is shoved back in that direction than in
+ a different one. fixed_directions facilitates that. Explained further
+ in the resize_gravity vs. fixed_directions section.
+ work_area_xinerama
+ This region is defined in the workspace and just cached here for
+ convenience. It is basically the area obtained by taking the current
+ xinerama, treating all partial struts as full struts, and then
+ subtracting all struts from the current xinerama region. Useful
+ e.g. for enforcing maximization constraints.
+ entire_xinerama
+ Just a cache of the rectangle corresponding to the entire current
+ xinerama, including struts. Useful e.g. for enforcing fullscreen
+ constraints.
+ usable_screen_region
+ The set of maximal/spanning rectangles for the entire screen; this
+ region doesn't overlap with any struts and helps to enforce
+ e.g. onscreen constraints.
+ usable_xinerama_region
+ The set of maximal/spanning rectangles for the current xinerama; this
+ region doesn't overlap with any struts on the xinerama and helps to
+ enforce e.g. the on-single-xinerama constraint.
+Gory details of resize_gravity vs. fixed_directions
+Note that although resize_gravity and fixed_directions look similar, they
+are used for different purposes:
+ - resize_gravity is only for resize operations and is used for
+ constraints unrelated to keeping a window within a certain region
+ - fixed_directions is for both move and resize operations and is
+ specifically for keeping a window within a specified region.
+Examples of where each are used:
+ - If a window is simultaneously moved and resized to the southeast corner
+ with SouthEastGravity, but it turns out that the window was sized to
+ something smaller than the minimum size hint, then the size_hints
+ constraint should resize the window using the resize_gravity to ensure
+ that the southeast corner doesn't move.
+ - If an application resizes itself so that it grows downward only (which
+ I note could be using any of three different gravities, most likely
+ NorthWest), and happens to put the southeast part of the window under a
+ partial strut, then the window needs to be forced back on screen.
+ (Yes, shoved onscreen and not clipped; see bug 136307). It may be the
+ case that moving the window to the left results in less movement of the
+ window than moving the window up, which, in the absence of fixed
+ directions would cause us to chose moving to the left. But since the
+ user knows that only the height of the window is changing, they would
+ find moving to the left weird (especially if this were a dialog that
+ had been centered on its parent). It'd be better to shove the window
+ upwards so we make sure to keep the left and right sides fixed in this
+ case. Note that moving the window upwards (or leftwards) is probably
+ totally against the gravity in this case; but that's okay because
+ gravity typically assumes there's more than enough onscreen space for
+ the resize and we only override the gravity when that assumption is
+ wrong.
+For the paranoid, a fixed directions might give an impossible to fulfill
+constraint (I don't think that's true currently in the code, but I haven't
+thought it through in a while). If this ever becomes a problem, it should
+be relatively simple to throw out the fixed directions when this happens
+and rerun the constraint. Of course, it might be better to rethink things
+to just avoid such a problem.
+The nitty gritty of what gets fixed:
+ User move:
+ in x direction - y direction fixed
+ in y direction - x direction fixed
+ in both dirs. - neither direction fixed
+ User resize: (note that for clipping, only 1 side ever changed)
+ in x direction - y direction fixed (technically opposite x side fixed too)
+ in y direction - x direction fixed (technically opposite y side fixed too)
+ in both dirs. - neither direction fixed
+ App move:
+ in x direction - y direction fixed
+ in y direction - x direction fixed
+ in both dirs. - neither direction fixed
+ App resize
+ in x direction - y direction fixed
+ in y direction - x direction fixed
+ in 2 parallel directions (center side gravity) - other dir. fixed
+ in 2 orthogonal directions (corner gravity) - neither dir. fixed
+ in 3 or 4 directions (a center-like gravity) - neither dir. fixed
+ Move & resize
+ Treat like resize case though this will usually mean all four sides
+ change and result in neither direction being fixed
+ Note that in all cases, if neither direction moves it is likely do to a
+ change in struts and thus neither direction should be fixed despite the
+ lack of movement.