Made EndFrame() assertion for key modifiers being unchanged during the frame more lenient. (#3575)

docs/CHANGELOG.txt

@@ -71,6 +71,9 @@ Other Changes:
 - Misc: Replaced UTF-8 decoder by branchless one by Christopher Wellons (30~40% faster). [@rokups]
   Super minor fix handling incomplete UTF-8 contents: if input does not contain enough bytes, decoder
   returns IM_UNICODE_CODEPOINT_INVALID and consume remaining bytes (vs old decoded consumed only 1 byte).
+- Misc: Made EndFrame() assertion for key modifiers being unchanged during the frame (added in 1.76) more
+  lenient, allowing full mid-frame releases. This is to accommodate the use of mid-frame modal native
+  windows calls, which leads backends such as GLFW to send key clearing events on focus loss. (#3575)
 - Backends: OpenGL3: Use glGetString(GL_VERSION) query instead of glGetIntegerv(GL_MAJOR_VERSION, ...)
   when the later returns zero (e.g. Desktop GL 2.x). (#3530) [@xndcn]
 - Backends: OpenGL3: Backup and restore GL_PRIMITIVE_RESTART state. (#3544) [@Xipiryon]

imgui.cpp

@@ -6905,9 +6905,13 @@ static void ImGui::ErrorCheckEndFrameSanityChecks()
 
     // Verify that io.KeyXXX fields haven't been tampered with. Key mods should not be modified between NewFrame() and EndFrame()
     // One possible reason leading to this assert is that your backends update inputs _AFTER_ NewFrame().
-    const ImGuiKeyModFlags expected_key_mod_flags = GetMergedKeyModFlags();
-    IM_ASSERT(g.IO.KeyMods == expected_key_mod_flags && "Mismatching io.KeyCtrl/io.KeyShift/io.KeyAlt/io.KeySuper vs io.KeyMods");
-    IM_UNUSED(expected_key_mod_flags);
+    // It is known that when some modal native windows called mid-frame takes focus away, some backends such as GLFW will
+    // send key release events mid-frame. This would normally trigger this assertion and lead to sheared inputs.
+    // We silently accommodate for this case by ignoring/ the case where all io.KeyXXX modifiers were released (aka key_mod_flags == 0),
+    // while still correctly asserting on mid-frame key press events.
+    const ImGuiKeyModFlags key_mod_flags = GetMergedKeyModFlags();
+    IM_ASSERT((key_mod_flags == 0 || g.IO.KeyMods == key_mod_flags) && "Mismatching io.KeyCtrl/io.KeyShift/io.KeyAlt/io.KeySuper vs io.KeyMods");
+    IM_UNUSED(key_mod_flags);
 
     // Report when there is a mismatch of Begin/BeginChild vs End/EndChild calls. Important: Remember that the Begin/BeginChild API requires you
     // to always call End/EndChild even if Begin/BeginChild returns false! (this is unfortunately inconsistent with most other Begin* API).